Hydrogen is an attractive fuel for high-altitude, long-duration aircraft because it contains about 2.8 times the energy per pound as compared with traditional hydrocarbon fuels. In aircraft applications requiring very long-range or endurance, the high specific energy of hydrogen may be a key enabler. However, practical considerations have largely prevented its use. While the specific energy of hydrogen is very high, the energy per unit volume is comparatively low.
In order to take advantage of the high specific energy of hydrogen, the associated tanks are preferably light weight—ideally being just a small fraction of the weight of the stored hydrogen (and preferably on the order of 10% to 25%). Typical tanks for storing compressed gaseous hydrogen have a weight of about 10 to 20 times that of the hydrogen stored, and are not likely practical for high-altitude, long-duration aircraft. Liquid hydrogen tanks for large rocket boosters have weights in the range of 10% of the hydrogen carried. However, these tanks are not intended for extended-duration storage (such as 1-2 weeks). They also benefit from the large-scale of the tanks, which is impractical for typical embodiments of a high-altitude, long-duration aircraft.
Liquid hydrogen powered high-altitude long-endurance aircraft will typically require tanks with sufficient insulation to prevent complete boil-off for one to two weeks. An anticipated capacity of an individual tank might range from 100 to 2000 pounds of liquid hydrogen, depending on the configuration and size of the airplane.
Smaller liquid hydrogen tanks have been demonstrated for some automotive applications. A very low heat leak rate is required for automotive uses, as vehicles are often stationary without any fuel being used, and it is desired that hydrogen not escape from the vehicle's tank over several days of inactivity. These tanks are typically very heavy (e.g., one system that stores 10 pounds of liquid hydrogen may weigh 200 pounds). Automotive safety standards also dictate a level of structural crash worthiness that would not typically be needed for a manned or unmanned aircraft.
The method of insulating a tank must deal with several types of heat transfer: conduction through solids, conduction and convention through gas, and radiation. Most methods of effecting high-performance insulation rely on a vacuum to nearly eliminate the conduction and convection gas heat transfer. Solid conduction is reduced by having the insulated tanks supported in the vacuum by structural supports of high-strength to conductivity ratio (e.g., stainless steel, glass fiber, or Dacron fiber). Radiant heat transfer is minimized by radiation shields (such as multi-layered insulation or opacified powders) and/or by polished highly reflective surfaces on the inner and outer walls of the vacuum chamber, as described in U.S. Pat. No. 6,347,719.
Accordingly, there has existed a need for an aircraft cryogenic storage tank that can provide for long-duration storage, and be characterized by a low weight. Moreover, this tank needs to operate in conjunction with other aircraft systems to provide cryogenic fuel at rates that meet fuel requirements, and using systems that maximize the overall efficiency of the aircraft. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.